Radiation curable compositions containing acrylic acid ester groups are mainly cured by exposure to ultraviolet light (UV). For a radiation curable composition, a photoinitiator is necessary, which forms radicals under irradiation with photons, and initiates free-radical polymerization, which then leads to a hardening (curing) of a liquid coating or ink.
In the last years, UV light emitting diodes (UV-LED) have become more attractive as the power output capabilities considerably increased for UV-LED's of wavelengths of 365-405 nm. The UV-LED bulbs do not generate ozone, in contrast to the typical UV-bulbs, require less energy, and exhibit a longer lifetime. Moreover, an additional advantage for UV-LED systems over the currently used UV-bulbs and UV-energy saving bulbs is the absence of mercury in the bulb. Therefore, UV-LED's are regarded as a “green” solution over typical mercury containing UV-bulbs.
Benzophenone derivatives (e.g. in CN103064251), or α-diketones such as campherquinone (in e.g. EP1245219) have been proposed as LED initiators. But the most widely used LED initiators are thioxanthone derivatives (e.g. in US2013176370) and acylphosphine oxide derivatives (e.g. in US2005234145).
In order to enable cure with UV-LED bulbs in a pigmented coating, such as a printing ink for example, ink makers typically increase the level of photoinitiators and sensitizers that absorb long wavelengths, such as acylphosphine or thioxanthone (TX) derivatives. Photoiniators that absorb long wavelengths are able to catch more light from UV-LED emissions, emitting at 365-405 nm, generating more radicals formed to promote curing of coatings and inks in depth, and especially on the surface. Surface curing is a challenge when using UV-LED, because short wavelengths irradiation is missing. Absorption of UV radiation of short wavelengths typically promotes surface cure.
However, using a high level of photoinitiators has the disadvantage that the ink or coating is filled with many small photoinitiator molecules which can migrate. Migration of small molecule photoinitiators causes concern about toxicology, odor, or off taste in food packaging applications.
Low molecular weight thioxanthone (TX) derivatives, such as isopropyl thioxanthone, tend to migrate and are reported to give unstable yellowing behavior (e.g. mentioned in WO2013/164394). Acyl phosphine oxide initiators can produce an odor, and because they are phosphor-organic compounds and type-I photoinitiators, can lead to splitting products. Splitting products can cause health and safety problems.
As a solution, higher molecular weight oligomeric and polymeric photoinitiators are proposed in the art to minimize migration in inks [“Industrial Photoinitiators”, CRC press London 2010]. Commercially available materials are, for example, oligomeric thioxanthone derivatives such as Omnipol TX (IGM resin) and Genopol TX (Rahn group). The UV absorption of these materials matches the UV emission of the UV-LED bulbs at 365-395 nm. However, usually such thioxanthone materials are not solely able to form enough radicals under irradiation, and require additional synergists and co-initiators, such as benzophenones and amines, in order to be effective. The synergists and co-initiators must be low migration products as well, which makes the photoinitiator blend sophisticated and costly.
In the last decades, water-based UV-curable systems have become a growing field, as water can reduce viscosity of UV-curable compositions, which makes it attractive for UV-flexographic, UV-gravure, and UV-digital printing applications. Moreover, if water can replace a part of an organic material in a formulation, it makes the whole formulation more sustainable. However, there are very limited water-soluble materials described which can be used as LED photoinitiators. Usually only aqueous dispersions, such as Irgacure 819Dw (BASF company) are described in the prior art, and therefore there is a high demand for water-soluble LED photoinitiator materials.
Additionally, due to legislation initiatives to phase out mercury containing UV-bulbs in electric and electronic equipment, there is a strong need to develop new photoinitiators curable with UV-LED light, for environmentally friendly applications, such as, for example, low migration inks and water-based inks and coatings.
DE 2709580 (U.S. Pat. No. 4,323,700) describes a process for preparing benzophenone derivatives from 3-phenylphthalide derivatives with the use of at least one oxidizing agent.
U.S. Pat. No. 4,431,840 discloses substituted 2-benzoylbenzoic acids useful as intermediates for preparing chromogenic phthalide compounds.
CN 101747654 discloses a method of preparing benzofuran ketone compounds containing indoles as substituent groups. The compounds are useful as electronic supplying colorless dye, and can be used as thermal-pressure sensitive color couplers in the paper industry.
JP H07-196929 discloses phthalide compounds useful as an electron-donating leuco dye for recording material having a color-developing part.
US 2008/0196176 discloses leuco colorants for use as consumer product additives to indicate a product function by color change, make visual effects, or to provide latent or delayed color generation.
U.S. Pat. No. 4,535,172 discloses 3-aryl or heteroaryl-3-alkoxy-, phenoxy-, alkylthio-, or phenylthiophthalides useful as color formers.
WO 2013/164394 discloses substituted derivatives of alpha-di-ketones which can be used as photoinitiators in LED photocuring.
US 2013/0176370 discloses an LED radiation curable composition comprising a thioxanthone derivative as a diffusion-hindered photoinitiator.
Therefore, the object of the present invention is to address the aforementioned technical shortcomings, and provide photoinitiators that preferably absorb in the region between 365 nm and 420 nm, exhibiting high photochemical reactivity and good surface cure, have minimal odor or odorous splitting products, and have an increased molecular weight for a low risk of migration.